Gasification for synthetic liquid fuel production
past, present, and future
R. Luque1; J.G. Speight2 1 University of Córdoba, Córdoba, Spain
2 CD&W Inc., Laramie, WY, USA
Abstract
Projections indicate that fossil fuels, such as coal and petroleum, will dominate the energy market for at least 50 years before biomass and other forms of alternative energy take hold. While the environmental issues resulting from the use of fossil fuel resources are undeniable, gasification and related technologies ameliorate the effects of fossil fuel combustion on acid rain deposition, urban air pollution, and global warming, and, as a result, they must be vigorously pursued. The recent emergence of new energy strategies indicate that society has begun to recognize the need to address fossil fuel use and its environmental impact.
13.1 Introduction
Projections indicate that fossil fuels, such as coal and petroleum, will continue to dominate the energy market for at least 50 years before biomass and other forms of alternative energy take hold (Speight, 2011a, 2011b, 2013a, 2013b). Furthermore, some authorities estimate that the era of fossil fuels will near its end when cumulative production reaches 85% of the initial total reserves (Hubbert, 1962). These claims may or may not have some merit. In fact, the relative scarcity of petroleum, as compared to a few decades ago, is real, but it seems likely that the remaining reserves will continue to provide an adequate supply of energy to the world for several decades (Banks, 1992; Krey et al., 2009; MacDonald, 1990; Martin, 1985; Speight, 2011c, 2013a, 2013b, 2014). The environmental issues that result from fossil fuel use are undeniable, however, and they require serious and continuous attention.
Technologies that ameliorate the effects of fossil fuel combustion on acid rain deposition, urban air pollution, and global warming must be vigorously pursued (Bending et al., 1987; Vallero, 2008). This is a challenge that must not be ignored, as the effects of acid rain on the soil and water leave no doubt about the need to control its causes (Mohnen, 1988). Indeed, the recent emergence of new energy strategies and research and development programs indicate that society has begun to recognize the need to address fossil fuel use and its environmental impacts (Stigliani & Shaw, 1990; United States Department of Energy, 1990; United States General Accounting Office, 1990).
While regulations on greenhouse gases, such as carbon dioxide (CO2), would be an immediate hurdle to the deployment of coal plants, gasification plants are better able to deal with carbon dioxide. However, with the continued uncertainty of carbon dioxide regulation, the industry is reluctant to make large investments in projects with high emissions of carbon dioxide because a cost-effective solution for reducing such emissions is not yet available. Nevertheless, the need to reduce greenhouse gas emissions can encourage the use of gasification in the long run because the carbon dioxide from a gasification plant is more amenable to capture.
As new technologies are developed, emissions may be reduced by repowering, a process through which aging equipment is replaced by more advanced and efficient substitutes. Such repowering might involve exchanging an aging unit for a newer combustion chamber, such as the atmospheric fluidized-bed combustor or the pressurized fluidized-bed combustor.
Indeed, many countries have recognized the vast quantity of atmospheric pollutants produced by fossil fuel use, and they have began to institute industrial emission standards. For a substance such as sulfur dioxide, the various standards are not only very specific but will become more stringent with the passage of time. In addition, heavy fines and jail terms may be issued to any pollution-minded miscreants who seek to flaunt the laws (Vallero, 2008). Nevertheless, increasing global fossil fuel use will require more stringent approaches to environmental protection than humankind has ever implemented. The need to protect the environment is strong.
13.2 Applications and products
Hydrogen and carbon monoxide, the major components of synthesis gas, are the basic building blocks of a number of other products, including fuels, chemicals, and fertilizers. In addition, a gasification plant can be designed to produce more than one product at a time (coproduction or polygeneration), such as electricity and chemicals (e.g., methanol or ammonia).
13.2.1 Chemicals and fertilizers
The process of producing energy through gasification has been in use for more than 100 years. Gasification was initially developed in the 1800s to produce town gas for lighting and heating, before being replaced by electricity and natural gas. It then continued to be used in blast furnaces. The gasification of coal was more significant in the production of synthetic chemicals, however, and it has been serving this function since the 1920s. The concept is now being considered as a means of producing much-needed chemicals, with the added benefit that low-value carbonaceous and hydrocarbonaceous feedstocks can also be gasified in a large chemical reactor. The resulting synthesis gas is cleansed and then converted into high-value products such as synthetic fuels, chemicals, and fertilizers.
Typically, the chemical industry uses gasification to produce methanol, as well as a variety of other chemicals, such as ammonia and urea, which form the foundation of nitrogen-based fertilizers and a variety of plastics. The majority of the world’s operating gasification plants are designed to produce chemicals and fertilizers.
13.2.2 Substitute natural gas
Gasification can also be used to convert coal into substitute natural gas (SNG) by using a methanation reaction in which the coal-based synthesis gas, which ismostly carbon monoxide and hydrogen, can be converted to methane.
Also called synthetic natural gas, SNG is an artificially produced version of natural gas that can be derived from coal, biomass, petroleum coke, or solid waste. The carbon‐containing mass can be gasified, and the resulting synthesis gas converted to methane, the major component of natural gas. There are several advantages associated with producing SNG. In times when natural gas is in short supply, SNG from coal could be a major driver for energy security by diversifying energy options and reducing imports of natural gas, thus helping to stabilize fuel prices.
Biomass and other low-cost feedstocks, such as municipal waste, can also be used along with coal to produce SNG. The use of biomass would reduce greenhouse gas emissions, as biomass is a carbon‐neutral fuel. In addition, the development of SNG technology would also boost the other gasification‐based technologies, including hydrogen generation and the integrated gasification combined cycle (IGCC).
As it is identical to conventional natural gas (methane, CH4), SNG can be transported in existing natural gas pipeline networks and used to generate electricity, produce chemicals and fertilizers, or heat homes and businesses. For many countries that lack natural gas resources, SNG enhances domestic fuel security by displacing imported natural gas that is likely to be supplied in the form of liquefied natural gas.
13.2.3 Hydrogen for petroleum refining
The use of hydrogen in thermal processes is perhaps the single most significant advance in refining technology during the twentieth century, and hydrogen is now employed in most refineries. In fact, now and in the future, refineries must deal with the changing availability of crude oil feedstocks and the conversion of these feedstocks into refined, transportation fuels, while complying with increasingly stringent clean fuel regulations. Refineries must also adjust to the decreasing heavy fuel oil demand and increasing supply of heavy, high-sulfur crude oils. Hydrogen network optimization can allow refineries to address clean fuel trends, to meet growing demands for transportation fuel, and to profit from their crudes (Long, Picioccio, & Zagoria, 2011). A key element of a refinery’s hydrogen network analysis involves the capture of hydrogen in its fuel streams in a manner that extends its flexibility and processing options. Thus, innovative hydrogen network optimization will be a critical factor influencing refineries’ future operational flexibility and profitability in a world of shifting crude feedstock supplies and ultra-low-sulfur gasoline and diesel fuel.
The process of upgrading heavy oils, residua, and related feedstocks evolved out of hydrodesulfurization processes (Ancheyta & Speight, 2007; Rana, Sámano, Ancheyta, & Diaz, 2007; Speight, 2014). In the early days, the goal was desulfurization, but the processes were later adapted to a 10-30% partial conversion operation. This new process was intended to achieve desulfurization and obtain low-boiling fractions simultaneously, by increasing severity in the operating conditions. However, as refineries have evolved and feedstocks have changed, refining heavy feedstocks has become a major issue for refineries, and several process configurations have emerged to accommodate the heavy feedstocks (Khan & Patmore, 1997; Speight, 2011a, 2014; Speight & Ozum, 2002).
As one of the two major components of synthesis gas, hydrogen is used to produce high-quality gasoline, diesel fuel, and jet fuel that meet the requirements for clean fuels in state and federal clean air regulations. Hydrogen is also used to upgrade heavy crude oil and tar sand bitumen. Refineries can gasify low-value residuals, such as petroleum coke, asphalts, tars, and some oily wastes from the refining process, to generate both the required hydrogen and the power and steam needed to run the refinery.
Thus, the gasification of petroleum residua, petroleum coke, and other feedstocks, such as biomass (Speight, 2008, 2011a, 2011b, 2014), to produce hydrogen and power may become an attractive option for refiners. The premise that the gasification section of a refinery will be the garbage can for deasphalter residues, high-sulfur coke, and other refinery wastes is worthy of consideration. Other processes such as ammonia dissociation, steam-methanol interaction, or electrolysis are also available for hydrogen production, but economic factors and feedstock availability affect the choice between processing alternatives.
13.2.4 Transportation fuels
Gasification is the foundation for converting coal and other solid feedstocks and natural gas into transportation fuels, such as gasoline, ultraclean diesel fuel, jet fuel, naphtha, and synthetic oils. Two options are available for converting carbonaceous feedstocks to motor fuels via gasification.
In the first option, the synthesis gas undergoes an additional process, the Fischer-Tropsch reaction (FT), to convert it to a liquid petroleum product. The FT process, with coal as a feedstock, was invented in the 1920s. Germany used FT-based technology during World War II, and it has been utilized in South Africa for decades. Currently, Malaysia and the Middle East also use FT processes with natural gas as the feedstock. In the second option, the methanol-to-gasoline process, the synthesis gas is first converted to methanol via a commercially used process, and the methanol is then converted to gasoline by reacting it over catalysts.
FT synthesis produces hydrocarbons of different chain lengths from a gaseous mixture of hydrogen and carbon monoxide. The higher-molecular-weight hydrocarbons can be hydrocracked to form diesel of excellent quality, among other products. The fraction of short-chain hydrocarbons is used in a combined-cycle plant with the remainder of the synthesis gas. As a result, the transportation sector will increasingly rely on fuel production through the gasification of biomass and the conversion of the gaseous products to FT fuels. However, large-scale, pressurized biomass gasification systems are necessary with particular attention given to the system’s gas-cleaning section.
13.2.5 Transportation fuels from tar sand bitumen
Tar sand deposits (oil sands deposits) can be found in many countries throughout the world, and these feedstocks may comprise more than 65% v/v of the total world oil reserve. The two largest deposits are in Canada and Venezuela. The Canadian tar sands are distributed in three major deposits thought to cover more than 54,000 square miles (140,000 km2), and the Alberta Energy and Utilities Board estimates that ~ 1.6 trillion barrels (1.6 × 1012 bbls) of crude oil equivalent are contained within the tar sand deposits of Canada. Of this amount, more than 170 billion barrels (170 × 109 bbls) are considered recoverable, but this amount is dependent on current oil prices.
Gasification is a commercially proven technology that can be used to convert petroleum coke into synthesis gas, and it is also being recognized as a means to economically generate hydrogen, power, and steam for tar sand operators in northeastern Alberta, Canada.
The tar sand deposits in Alberta are estimated to contain as much recoverable bitumen as the petroleum available from the vast oil fields of Saudi Arabia. However, converting the raw bitumen to saleable products requires extracting the bitumen from the sand and refining the separated bitumen to transportation fuels. The mining process requires massive amounts of steam to separate the bitumen from the sand, and the refining process demands large quantities of hydrogen to upgrade the raw distillates to saleable products. Residual materials from the bitumen-upgrading process include petroleum coke, deasphalted residua, vacuum residua, all of which contain unused energy that can released and captured for use by gasification. Traditionally, tar sand operators have utilized natural gas to produce the steam and hydrogen needed for the mining, upgrading, and refining processes.
Oil sands operators have most often utilized natural gas to produce the steam and hydrogen needed for the mining, upgrading, and refining processes. However, a number of operators will soon gasify coke to supply the necessary steam and hydrogen. Not only will gasification displace expensive natural gas as a feedstock, it will also enable the extraction of useable energy from what is otherwise a very low-value product (coke). In addition, traditional oil sand operations consume large volumes of water, but, with gasification, black water from the mining and refining processes can be recycled to the gasifiers, using a wet feed system, thus reducing fresh water usage and waste water management costs.
13.2.6 Power generation
Converting coal to power through gasification technology allows the continued use of domestic supplies of coal without the high level of air emissions associated with conventional coal-burning technologies. One of the advantages of the coal gasification technology is that it offers the polygeneration: coproduction of electric power, liquid fuels, and chemicals from hydrogen and the syngas generated from gasification.
Furthermore, an integrated gasification combined cycle power plant (IGCC power plant) combines the gasification process with a combined-cycle power block consisting of one or more gas turbines and a steam turbine. Clean synthesis gas is combusted in high-efficiency gas turbines to produce electricity. The excess heat from the gas turbines and from the gasification reaction is then captured, converted into steam, and sent to a steam turbine to produce additional electricity.
In the IGCC power plant, which is focused on power generation, the clean synthesis gas is combusted in high-efficiency gas turbines to generate electricity with very low emissions. The gas turbines used in these plants are slight modifications of proven, natural gas combined-cycle (NGCC) gas turbines that have been specially adapted for use with synthesis gas. For IGCC power plants that include carbon dioxide capture, these gas turbines are adapted to operate on synthesis gas with higher levels of hydrogen. Although state-of-the-art gas turbines are commercially ready for the high-hydrogen synthesis gas, there is a movement to develop the next generation of even more efficient gas turbines ready for carbon dioxide capture-based IGCC power plants.
The heat recovery steam generator (HRSG) captures heat in the hot exhaust from the gas turbines and uses it to generate additional steam that is used to make more power in the steam turbine portion of the combined-cycle unit. In most IGCC power plant designs, steam recovered from the gasification process is superheated in the HRSG to increase the overall efficiency output of the steam turbines. As a result, the IGCC combination, which includes a gasification plant, two types of turbine generators (gas and steam), and the HRSG, is clean and efficient.
Biomass fuel producers, coal producers, and, to a lesser extent, waste companies are enthusiastic about supplying cogasification power plants, and these producers realize the benefits of cogasification with alternate fuels. Cogasification technology can capitalize on a reliable coal supply with gate-fee waste, as well as biomass that allows the use of a larger plant than could be supplied just with waste and biomass. In addition, the technology offers a future option for hydrogen production and fuel development in refineries. In fact, when hydrogen is particularly valuable, oil refineries and petrochemical plants provide opportunities for gasifiers (Speight, 2011a).
13.2.7 Waste-to-energy gasification
Municipalities are spending millions of dollars each year to dispose of solid waste that, in fact, contains valuable unused energy. In addition to the expense of collecting this waste, they must also contend with increasingly limited landfill space, the environmental impacts of landfilling, and stringent bans on the use of incinerators. As a result of these challenges, municipalities are increasingly looking to gasification as a solution, transforming waste into energy rather than burying it.
The traditional waste-to-energy plant, which is based on mass-burn combustion on an inclined grate, has low public acceptability despite the very low emissions achieved over the last decade with modern flue gas cleanup equipment. This lack of popular support for mass-burn operations has led to companies having difficulty obtaining planning permissions to construct the new waste-to-energy plants that are needed. After much debate, various governments have allowed options for advanced waste conversion technologies (gasification, pyrolysis, and anaerobic digestion), but these same governments will only give credit to the proportion of electricity generated from nonfossil waste.
Gasification can convert municipal, construction, and demolition wastes that cannot otherwise be recycled into electric power or other valuable products, such as chemicals, fertilizers, and SNG. Instead of paying to dispose of these wastes, municipalities are generating income by using the wastes as valuable feedstocks for a gasifier. Gasifying municipal and other waste streams reduces the need for landfill space, decreases the generation of methane (a potent greenhouse gas) generated by bacterial action as the landfill matures, and reduces the potential for groundwater contamination from landfill sites.
Coutilization of waste and biomass with coal may provide economies of scale that help achieve the above-identified policy objectives at an affordable cost. In some countries, governments propose cogasification processes as being well suited for community-sized developments, suggesting that waste should be dealt with in smaller plants serving towns and cities, rather than moved to large, central plants, thus satisfying the so-called proximity principal, or the tendency to band similar entities together to achieve a common goal.
Municipalities’ use of gasifiers to dispose of waste and create energy is almost a return to the days when gasification first became commercially available and every small town had a gasification plant to produce gas (hence the name town gas) for heating and lighting purposes.
However, it is important to add that gasification does not compete with recycling. In fact, gasification complements existing recycling programs through the creation of an added-value product (energy). Many materials, including a wide range of plastics, cannot currently be recycled or recycled any further and are ideal candidates for feedstocks in the gasification process. As the amount of waste generated increases in line with an increase in the population, and recycling rates increase to the point of overburdening the system, gasification will alleviate any potential bottlenecks through the generation of energy.
13.2.8 Biomass gasification
Biomass comprises a wide range of materials, including energy crops such as switch grass and miscanthus, agricultural sources such as corn husks, wood pellets, lumbering and timbering wastes, yard wastes, construction and demolition waste, and biosolids (treated sewage sludge). Gasification helps recover the energy locked in these materials. Gasification can convert biomass into electricity and products, such as ethanol, methanol, fuels, fertilizers, and chemicals. Thus, in addition to using the traditional coal, petroleum coke, and other traditional feedstocks, gasifiers can be designed to convert biomass.
Biomass usually has a high moisture content (along with carbohydrates and sugars). The presence of high levels of moisture in the biomass reduces the temperature inside the gasifier, which then reduces the efficiency of the gasifier. Therefore, many biomass gasification technologies require that the biomass be dried to reduce its moisture content before it is fed into the gasifier.
As with many solid feedstocks, biomass can come in a range of sizes. In many biomass gasification systems, the biomass must be processed to a uniform size or shape so that it might be fed into the gasifier at a consistent rate and to ensure that as much of the biomass is gasified as possible. However, beyond the issue of biomass availability, including the seasonal factors associated with many of the biomass feedstocks, another major concern is that more energy is expended in collecting and preparing the biomass than is generated through actual gasification, and technical hurdles to biomass use remain. In general, many countries seem to be increasingly using biomass feedstocks in response to environmental and regulatory factors, rather than free-market forces. Without tax credits or similar incentives, biomass is unlikely to be used as a base-load feedstock, and market entry is likely to involve cogasification or other blended use (Clayton, Stiegel, & Wimer, 2002).
Most biomass gasification systems use air as a gasifying agent, instead of oxygen, which is typically used in large-scale industrial and power gasification plants. Gasifiers that use oxygen require an air separation unit to provide the gaseous or liquid oxygen, and air separation is usually not cost-effective at the smaller scales used in biomass gasification plants. Thus, air-blown gasifiers use the oxygen in the air for the gasification reactions.
In general, biomass gasification plants are much smaller than the typical coal or petroleum coke gasification plants used in the power, chemical, fertilizer, and refining industries. As such, they are less expensive to build and have a smaller facility footprint. While a large industrial gasification plant may take up 150 acres of land and process 2500-15,000 tons per day of feedstock such as coal or petroleum coke, the smaller biomass plants typically process 25-200 tons of feedstock per day and take up < 10 acres.
Currently, most ethanol in the United States is produced from the fermentation of corn. Vast amounts of corn, and the land, water, and fertilizer required to grow it, are needed to produce ethanol. As more corn is being used, some observers have raised concerns about the decreasing availability of food corn. Gasifying biomass, such as corn stalks, husks, and cobs, and other agricultural waste products, to make ethanol and synthetic fuels such as diesel and jet fuel can help break this energy-food competition.
Biomass, such as wood pellets, yard wastes, and crop wastes, and energy crops, such as switch grass and waste from pulp and paper mills, can be used to produce ethanol and synthetic diesel. The biomass is first gasified to produce syngas (synthesis gas) and then converted via catalytic processes to these downstream products.
Each year, municipalities spend millions of dollars collecting and disposing of wastes, such as yard wastes (grass clippings and leaves) and construction and demolition debris. While some municipalities compost yard wastes, composting requires a separate collection by a city, which is an expense many cities just can’t afford. Yard waste and the construction and demolition debris can also take up valuable landfill space, shortening the life of a landfill. With gasification, this material is no longer a waste, but a feedstock for a biomass gasifier. And, as opposed to paying to dispose of and manage a waste for years in a landfill, using it as a feedstock reduces disposal costs and landfill space and converts wastes into power and fuels.
Thus, the benefits of biomass gasification include (1) converting what would otherwise be a waste product into high-value products, (2) reducing the need for landfill space for disposal of solid wastes, (3) decreasing methane emissions from landfills, (4) reducing the risk of groundwater contamination from landfills, and (5) producing ethanol from nonfood sources. Thus, municipalities, as well as the paper and agricultural industries, would be well advised to use gasification to reduce the disposal costs associated with these wastes, as well to produce electricity and other valuable products from these waste materials. While still relatively new, biomass gasification shows a great deal of promise. A key advantage of gasification is that it can convert nonfood biomass materials, such as corn stalks and wood wastes, to alcohols. Furthermore, unlike traditional process for making alcohols, biomass gasification does not remove food-based biomass, such as corn, from the economy.
13.3 Environmental benefits of gasification-based systems
The careless combustion of fossil fuels can account for the large majority of the sulfur oxides and nitrogen oxides released to the atmosphere. If a technology can succeed in reducing the amounts of these gases in the atmosphere, it should also succeed in reducing the amounts of urban smog, those odorous brown and gray clouds that are often visible over cityscapes and lead to the deposition of acid rain:
In the United States, a growing awareness of the environmental impacts of fossil use has led the government to adopt the Clean Fossil Fuels Program to facilitate the development of pollution abatement technologies. This new attention to pollution has also led to successful partnerships between government and industry (United States Department of Energy, 1993). In addition, new laws, such as the 1990 Clean Air Act Amendments in the United States, have the potential to encourage the controlled clean use of fossil fuels (Stensvaag, 1991; United States Congress, 1990). There will be a cost associated with clean use, but industry is supportive of the measure and confident that the goals can be met.
Besides fuel and product flexibility, gasification-based systems offer significant environmental advantages over competing technologies, particularly coal-to-electricity combustion systems. Gasification plants can readily capture carbon dioxide, the leading greenhouse gas, much more easily and efficiently than coal-fired power plants. In many instances, this carbon dioxide can be sold, creating additional value from the gasification process.
Carbon dioxide captured during the gasification process can be used to help recover oil from otherwise depleted oil fields. The Dakota Gasification plant in Beulah, North Dakota, captures its carbon dioxide, while making SNG, and it then sells the carbon dioxide for enhanced oil recovery. Since 2000, this plant sent its captured carbon dioxide via pipeline to EnCana’s Weyburn oil fields in Saskatchewan, Canada, where it is used for enhanced oil recovery. As a result, more than five million tons of carbon dioxide has been sequestered.
13.3.1 Carbon dioxide capture
In a gasification system, carbon dioxide can be captured using commercially available technologies, such as the water gas shift reaction, before it is vented into the atmosphere. Converting the carbon monoxide to carbon dioxide and capturing it prior to combustion is more economical than removing carbon dioxide after combustion, effectively “decarbonizing” or, at least, reducing the carbon in the synthesis gas.
Gasification plants manufacturing ammonia, hydrogen, fuels, or chemical products routinely capture carbon dioxide as part of the manufacturing process. According to the Environmental Protection Agency, the higher thermodynamic efficiency of the IGCC process minimizes carbon dioxide emissions, relative to the emissions of other technologies. IGCC plants offer a least-cost alternative for capturing carbon dioxide from a coal-based power plant. In addition, facilities using IGCC will experience a lower energy penalty than other technologies if carbon dioxide capture is required. While carbon dioxide capture and sequestration will increase the cost of all forms of power generation, an IGCC plant can capture and compress carbon dioxide at one-half the cost of a traditional pulverized coal plant. Other gasification-based options, including the production of motor fuels, chemicals, fertilizers, and hydrogen, have even lower carbon dioxide capture and compression costs, which will provide a significant economic and environmental benefit in a carbon-constrained world.
13.3.2 Lower air emissions
Gasification can achieve greater air emission reductions at lower cost than other forms of coal-based power generation, such as supercritical pulverized coal. Coal-based IGCC offers the lowest emissions of sulfur dioxide, nitrogen oxides, and particulate matter of any coal-based power production technology. In fact, a coal IGCC plant is able to achieve low air-emissions rates that approach those of a NGCC power plant. In addition, mercury emissions can be removed from an IGCC plant at one-tenth the cost of removal from a coal combustion plant. Technology exists today to remove more than 90% w/w of the volatile mercury from the synthesis gas in a coal-based gasification plant.
13.3.3 Solids generation
During gasification, virtually all of the carbon in the feedstock is converted to synthesis gas. The mineral material in the feedstock separates from the gaseous products, and the ash and other inert materials melt and fall to the bottom of the gasifier as a nonleachable, glasslike solid or other marketable material. This material can be used for many construction and building applications. In addition, more than 99% w/w of the sulfur can be removed using commercially proven technologies, prior to being converted into marketable elemental sulfur or sulfuric acid.
13.3.4 Reduced water use
Gasification uses ~ 14-24% v/v less water to produce electric power from coal, compared to other coal-based technologies, and water losses during operation are about 32-36% v/v less than the losses incurred by other coal-based technologies. This is a major issue in many countries, including the United States, where water supplies have already reached critical levels in certain regions.
13.4 A process for now and the future
Gasification differs from more traditional energy-generating process, in that it is not a combustion process, but rather a conversion process. Instead of the carbonaceous feedstock being wholly burned in air to create heat for the production of steam to drive turbines, the feedstock to be gasified is combined with steam and limited oxygen in a heated, pressurized vessel. The atmosphere inside the vessel is deficient in oxygen leading to a complex series of reactions in the feedstocks that produce synthesis gas. Moreover, using current technologies, the synthesis gas can be cleaned beyond current and proposed environmental regulatory requirements, as demonstrated by commercial chemical production plants that require ultraclean synthesis gas to protect the integrity of expensive catalysts. The clean synthesis gas can be combusted in turbines or engines using more efficient higher-temperature cycles than the conventional steam cycles associated with burning carbonaceous fuels, and as a result, the use of clean synthesis gas allows for possible efficiency improvements. Synthesis gas can also be used in fuel cells and fuel cell-based cycles with yet even higher efficiencies and exceptionally low emissions of pollutants.
Furthermore, one of the major challenges of the twenty-first century is finding a way to meet national and global energy needs, while minimizing the impact on the environment. There is extensive debate surrounding this issue, certain areas of focus have emerged: (1) production of cleaner energy from conventional fuel sources and alternative technologies, (2) use of energy sources that are environmentally sound and economically viable, (3) investment in a variety of technologies and resources to produce clean energy to meet energy needs. Gasification technologies will help to answer these challenges.
13.4.1 The process
As a time-tested, reliable and flexible technology, gasification will be an increasingly important component of this new energy equation, even leading petroleum refineries to evolve, as more gasification units are added to them (Speight, 2011a). Any investment in gasification will yield valuable future returns in clean, abundant, and affordable energy from a variety of sources (Speight, 2008, 2011b).
Gasification is an environmentally sound way to transform any carbon-based material, such as coal, refinery by-products, biomass, or even waste, to energy by producing synthesis gas that can be converted into electricity and valuable products, such as transportation fuels, fertilizers, SNG, or chemicals (Chadeesingh, 2011; Speight, 2013a).
Gasification has been used on a commercial scale for ~ 100 years by the coal, refining, chemical, refining, and lighting industries. It is currently playing an important role in meeting energy needs in many countries, and it will continue to play an increasingly important role as an economical manufacturing technology that produces clean, abundant energy. And, while gasification has typically been used in industrial applications, the technology is increasingly being adopted in smaller-scale applications to convert biomass and waste to energy and products.
Gasification is the cleanest, most flexible, and most reliable way to use fossil fuels and a variety of other carbonaceous or hydrocarbonaceous feedstocks. It can convert low-value materials into high-value products, such as chemicals and fertilizers, SNG, transportation fuels, electric power, steam, and hydrogen. The process can also be used to convert biomass, municipal solid waste, and other materials (that are normally burned as fuel) into a clean gas. In addition, gasification provides the most cost-effective means of capturing carbon dioxide, a greenhouse gas, when generating power with fossil fuel feedstock. Many countries also depend on high-cost imported petroleum and natural gas from politically unstable regions of the world, and gasification allows these countries to use of domestic resources to generate energy.
In fact, gasifiers can be designed to use a single material or a blend of feedstocks of the following types: (1) solids, such as coal, petroleum coke, biomass, wood waste, agricultural waste, household waste, and hazardous waste; (2) liquids, such as petroleum resids (including used or recovered road asphalts), tar sand bitumen, and liquid wastes from chemical plants and other processing plants; and (3) gas, such as natural gas or refinery and chemical processing off-gas.
The specific gasification technology used determines the output of the most sought-after products of the process, synthesis gas and hydrogen, and smaller quantities of methane, carbon dioxide, hydrogen sulfide, and water vapor are also produced, with 70-85% of the carbon in the feedstock typically converted into synthesis gas. The ratio of carbon monoxide to hydrogen depends in part upon the hydrogen and carbon content of the feedstock and the type of gasifier used, but this ratio can also be adjusted downstream of the gasifier through use of catalysts. This inherent flexibility of the gasification process means that it can produce one or more products from the same process.
Another benefit of gasification is carbon dioxide removal during the synthesis gas cleanup stage, using a number of proven commercial technologies (Mokhatab, Poe, & Speight, 2006; Speight, 2007). In fact, carbon dioxide is routinely removed in gasification-based ammonia, hydrogen, and chemical manufacturing plants. Gasification-based ammonia plants already separate and capture ~ 90% v/v of the carbon dioxide, and gasification-based methanol plants capture ~ 70% v/v of the carbon dioxide. Thus, the gasification process offers the most cost-effective and efficient means of capturing carbon dioxide during the energy production process.
Other by-products include slag – a glass-like product – composed primarily of sand, rock, and minerals originally contained in the gasifier feedstock. This slag is non-hazardous and can be used in roadbed construction, cement manufacturing, and the manufacture of roofing materials. Also, in most gasification plants, more than 99% w/w of the feedstock sulfur is removed and recovered either as elemental sulfur or sulfuric acid.
In addition, plasma gasification is increasingly being used to convert all types of waste, including municipal solid waste and hazardous waste, into electricity and other valuable products. Plasma is an ionized gas that is formed when an electrical charge passes through a gas. Plasma torches generate extremely high temperatures that can initiate and intensify gasification reactions, increasing the rate of those reactions and making gasification more efficient. The plasma system can also convert different types of mixed feedstocks, such as municipal solid waste and hazardous waste, removing the expensive step of sorting the feedstock by type before it is fed into the gasifier. These significant benefits make plasma gasification an attractive option for managing different types of wastes.
13.4.2 Refineries of the future
As it enters the twenty-first century, the petroleum refining industry is experiencing its greatest innovations driven by the increasing supply of heavy oils with decreasing quality, as well as the rapidly increasing demand for clean and ultraclean vehicle fuels and petrochemical raw materials. As feedstocks to refineries change, there must be an accompanying change in refinery technology. This change requires a movement from conventional means of refining heavy feedstocks that typically use coking technologies to more innovative processes, including hydrogen management, that will produce the maximum amounts liquid fuels from the feedstock and maintain emissions within environmental compliance (Davis & Patel, 2004; Lerner, 2002; Penning, 2001; Speight, 2008, 2011a).
To meet the challenges from changing restructured over the years from simple crude trends in product slate and the stringent distillation operations into increasingly specifications imposed by environmental complex chemical operations involving legislation, the refining industry in the near transformation of crude oil into a variety of future will become increasingly flexible and refined products with specifications that meet innovative with new processing schemes, users requirements.
During the next 20-30 years, the evolution of petroleum refining and refinery configurations will likely focus on process modification with some new innovations emerging (Speight, 2014). Predictably, the industry will move on to deep conversion of heavy feedstocks, higher hydrocracking and hydrotreating capacity, and more efficient processes.
High-conversion refineries will begin to use gasification to produce alternative fuels and to enhance equipment usage. When cost begins to prohibit the production of superclean transportation fuels using conventional technologies, refineries will also move toward gasification to meet the increasing demand for fuels synthesized from simple basic reactants (e.g., synthesis gas). FT plants and IGCC systems will also be integrated with or even into refineries, offering the advantage of high-quality products (Stanislaus et al., 2000). The Sasol refinery in South Africa provides an example of a facility to be centered on gasification technology (Couvaras, 1997). The refinery would produce synthesis gas to be used in manufacturing liquid fuels via FT processes.
In fact, gasification to produce synthesis gas can proceed from any carbonaceous material, including biomass. The inorganic components of the feedstock, such as metals and minerals, are trapped in an inert and environmentally safe form as char, which may have use as a fertilizer. Biomass gasification is therefore one of the most technically and economically convincing forms of energy generation for a potentially carbon-neutral economy.
A modified version of steam reforming, known as autothermal reforming, combines partial oxidation near the reactor inlet with conventional steam reforming further along the reactor, and as a result, it improves the overall reactor efficiency and increases the flexibility of the process. Partial oxidation processes using oxygen instead of steam also found wide application for synthesis gas manufacture, with the special feature that they could utilize low-value feedstocks such as heavy petroleum residua. In recent years, catalytic partial oxidation employing very short reaction times (milliseconds) at high temperatures (850-1000 °C, 1560-1830 °F) has been providing yet another approach to synthesis gas manufacture (Hickman & Schmidt, 1993).
As petroleum supplies decrease, the desirability of producing gas from other carbonaceous feedstocks will increase, especially in those areas where natural gas is in short supply. Natural gas costs are also likely to increase, allowing coal gasification to compete as an economically viable process. Current research, both in the laboratory and at pilot-plants, should lead to the invention of new process technology by the end of the century, thus accelerating the industrial use of coal gasification.
The conversion of the gaseous products of gasification into synthesis gas still requires additional steps after purification, but the gases produced during this process, such as carbon monoxide, carbon dioxide, hydrogen, methane, and nitrogen, can be used as fuels or as raw materials for chemical or fertilizer manufacture.
13.4.3 Economic aspects
As with any manufacturing unit, a gasification plant is capital intensive, but its operating costs can be lower than those faced by many other manufacturing processes or coal combustion plants. A gasification plant can use low-cost feedstocks, such as petroleum coke or high-sulfur coal, converting them into high-value products. So, it increases the use of available energy in the feedstocks, while reducing disposal costs. Ongoing research, development, and demonstration investment efforts show potential to substantially decrease current gasification costs even further, increasing the economic attractiveness of gasification.
In addition, gasification has a number of other significant economic benefits: (1) the principal gasification by-products (sulfur, sulfuric acid, and slag) are marketable; (2) gasification can produce a number of high-value products at the same time (cogeneration or polygeneration), helping a facility offset its capital and operating costs, while diversifying its risks; (3) gasification offers wide feedstock flexibility, because a gasification plant can be designed to vary the mix of the solid feedstocks or to run on natural gas or liquid feedstocks when desirable; (4) gasification units require less emission control equipment because they generate fewer emissions, further reducing the plant’s operating costs.
Investment in gasification also involves the construction, operation, and maintenance of large-scale plants, and as a result, it increases business for suppliers at home and abroad, while creating domestic jobs in sectors such as construction and machine operation that cannot be outsourced to overseas workers.
13.4.4 Market outlook
The forecast for growth of gasification capacity focuses on two areas: large-scale industrial and power generation plants and the small-scale biomass and waste-to-energy plants.
Worldwide, the capacity of gasification for industry and power generation is projected to grow 70% by 2015, with 81% of the growth occurring in developing markets. The prime movers behind this expected growth are the chemical, fertilizer, and coal-to-liquids industries in China, tar sands in Canada, polygeneration (hydrogen and power or chemicals) in the United States, and refining in Europe. In fact, China has focused on gasification as part of its overall energy strategy. The industrial and power gasification industry in the United States faces a number of challenges, however, including rising construction costs and uncertainty about policy-based incentives and regulations. Despite these challenges, the industrial and power gasification capacity in the United States is expected to grow.
A number of factors will contribute to this expansion. Volatile oil and natural gas prices will make low-cost and abundant domestic resources with stable prices increasingly attractive as feedstocks, and gasification processes will be able to comply with more stringent environmental regulations because their emission profiles are already substantially less than more conventional technologies.
In fact, there is a growing consensus that carbon dioxide management will be required in power generation and energy production. Given that the gasification process allows carbon dioxide to be captured in a cost-effective and efficient manner, it will be an increasingly attractive choice for the continued use of fossil fuels. In terms of the number of plants in the United States, the greatest growth is likely to occur in the biomass and waste-to-energy gasification areas. Because they are smaller in scale, these plants are easier to finance, easier to permit, and require less time to construct. In addition, municipal and state restrictions on landfills and incineration and a growing recognition that these materials contain valuable sources of energy are driving the demand for these plants.
Finally, a number of factors will contribute to the growing interest in waste and biomass gasification: (1) restrictions on landfill space, (2) efforts to reduce costs associated with waste management, (3) growing recognition that waste and biomass contain unused energy that can be captured and converted into energy and valuable products, (4) the availability of nonfood biomass for conversion into valuable energy products.
13.5 Conclusions
The most obvious issue with fossil fuel use relates to its effects on the environment. As technology evolves, the means to reduce the damage done by fossil fuel use also evolves, and the world is on the verge of adopting alternative energy sources. In the meantime, gasification offers alternatives to meet future fuel demand, while reducing potentially harmful emissions.
Recent policy aimed at tackling climate change and resource conservation, such as the Kyoto Protocol, the deliberations at Copenhagen in 2009, and the Landfill Directive of the European Union, stimulated the development of renewable energy and landfill diversion technology, thus providing renewed impetus for developing gasification technology. However, even though they are the fastest growing source of energy, renewable sources of energy will still represent only 15% of the world energy requirements in 2035 (up from the current estimation of 10%), and divesting from fossil fuels does not mean an end to environmental emissions. Petroleum, tar sand bitumen, coal, natural gas, and perhaps oil shale will still be dominant energy sources, and their use will grow at a relatively robust rate for at least the next two decades. These estimates provide a reality check for those hoping to implement clean technologies, and they should head it, if they hope to reduce greenhouse gas emissions, while satisfying future energy demands (EIA, 2013).
Gasification could now be proposed as a viable alternative solution for waste treatment with energy recovery. On the other hand, the gasification process still faces some technical and economic problems, mainly related to the highly heterogeneous nature of municipal solid wastes and related feeds and the relatively limited number of plants (~ 100) worldwide with continuous experience using this technology under commercial conditions. In the aggressive working environment of municipal solid waste management, with its uncompromising demand for reasonable cost, high reliability, and operational flexibility, it could be premature to indicate that gasification is the thermal processing strategy of the future, or even a strong competitor for combustion systems, at least for all waste-to-energy plants.
The success of any advanced thermal technology is determined by its technical reliability, environmental sustainability, and economic convenience. Around one hundred gasification-based waste-to-energy plants, mainly in Japan but now also in Korea and Europe, have logged years of continuous operation, demonstrating technical reliability. Environmental performance is also one of the greatest strengths of gasification technology, which often is considered a sound response to the increasingly restrictive regulations applied around the world, and independently verified emissions tests indicate that gasification is able to meet existing emissions limits and can have a great effect on the reduction of landfill use.
Economic aspects are probably the biggest obstacle to market penetration, because gasification-based waste-to-energy conversion tends to have operating and capital costs higher than those of conventional combustion-based waste-to-energy (in the order of about 10%), mainly as a consequence of the ash-melting system or the added complexity of the technology.
The evidence of the last decade or so indicates the convenience of constructing gasification plants having a capacity less than approximately 120,000 tons feedstocks per year. In order to achieve a wider market penetration, advanced gasification technologies must be able to provide cheaper synthesis and gas cleaning, while conveniently meeting defined specifications and obtaining higher electric energy conversion efficiencies. Nevertheless, the performance and experience of the commercial waste gasifiers in operation illustrate that gasification processes can indeed compete with conventional moving-grate or fluidized-bed combustion systems.